Optical network units are conventionally placed close to customers' premises, e.g. businesses or homes. These units are connected to central offices by optical fibers and/or copper pairs. Signals received from these fibers and copper pairs are converted into electrical signals which are then transmitted along relatively short electrical conductors to the customers' premises. Electrical signals, in the frequency range above the voice band, received from the customers are converted by the optical network units into optical signals which are transmitted by the optical fibers to the central office. Electrical signals in the voice band received from the customer are transmitted to the central office on the copper pairs. Normally it is expected that an optical network unit is situated adjacent to equipment known by various terms such as a joint wiring interconnect, outside wiring interface, serving area interconnect or feeder distribution interface. This equipment connects the optical network unit to the customer's line to transfer the voice, data, video, high definition broad band and XDSL signals to the customer.
Optical network units are located outside customers' premises whereby they are subject to ambient temperature, wind and humidity conditions. For protection against these conditions and against vandalism, a housing is required to provide suitable protection to the circuitry, connections and any other features within the housing and upon which an optical network unit depends for operation.
In order to provide for their functional requirements, there are various designs of optical network units. In one particular design, e.g. as shown in U.S. Pat. Nos. 5,828,807 and 5,982,972, a frame has two compartments each having an opening. One compartment houses the electronics and optics of the optical network unit, and the second compartment houses a drop terminal block or blocks of the unit. Doors are provided for covering and sealing the openings when the unit is in use. The doors have a gull-wing design which enables them to be rotated upwards when the interior of the optical network is to be accessed. Catches or struts are provided to hold the doors in open positions, thereby freeing the hands of a technician working on the unit with the unit mounted horizontally, in which position the doors rotate by horizontal axes. With the doors open, each door thus overhangs its opening with the intention of preventing rain from entering its associated compartment.
One problem that may exist with the structure as described in the above patents, is that the internal environment for the electronics and optical components, is not controlled. Since field replaceable line cards are intended to be serviced through manual exchange through the opening to the appropriate compartment, humidity that enters the compartment, when the door is opened, is trapped inside the compartment after the door is closed, to seal the compartment. This humidity may affect the performance and reliability of the product, as the humidity, either as airborne mist or as heat-condensed droplets of water, may cause corrosion of electronic components and/or shorting out of the power supply. This would interfere with electrical signalling and/or power distribution. Even when new line cards are inserted into the compartment, the expected operating time, as required by industry standards, may be compromised due to the undesirable humid internal environment. In addition, the direction of wind-driven rain may be subject to change under certain uncontrollable environmental conditions. As a result, the doors when in open condition may not always prevent rain from entering the compartment. Wind-driven rain entering the electronics and optical compartment during service has similar potential for compromising the reliability and performance of the unit through corrosion and shorting out, as does trapped humidity.
A further problem that may exist with the above type of design relates to how the design affords the opportunity for human error to occur during service. Extended down time may be due to the exchange of line cards, which do not require exchange, and which are mistakenly identified as inoperable line cards. This would result in unnecessary delays in returning an optical network unit to its desirable function. It has been shown that in different types of electronic equipment, unnecessarily exchanging correctly operating line cards, where incorrectly operating cards should be exchanged, is a real and practical problem. Upon such mistakes being made, the problem may become intensified, thereby leading to further delay, when upon providing power to the unit once again, it is still found to be operating incorrectly. Such unnecessary delays, apart from economical considerations, and wastage of technicians' time, are likely to lead to customers' dissatisfaction due to extended lack of service.